Supported biofilm apparatus and process

ABSTRACT

A membrane supported biofilm reactor uses modules having fine, hollow fibres, for example, made from dense wall Poly methylpentene (PMP) used in tows or formed into a fabric. In one module, one or more sheets of the fabric are potted into a module to enable oxygen containing gas to be supplied to the lumens of the hollow fibres. Various reactors and processes, for example to treat wastewater, using such modules are described. Mechanical, chemical and biological methods are used to control the thickness of the biofilm.

[0001] This application is (1) a continuation-in-part of U.S. Ser. No.10/777,204 filed Feb. 13, 2004 which is an application claiming thebenefit under 35 USC 119(e) of U.S. Provisional Patent ApplicationSerial No. 60/447,025 filed Feb. 13, 2003; (2) an application claimingthe benefit under 35 USC 119(e) of U.S. Provisional Application SerialNo. 60/496,178 filed Aug. 18, 2003; and (3) a continuation of a PCTapplication, not yet assigned a serial number, filed Feb. 13, 2004. Thisapplication also claims priority from Canadian Patent Application Nos.2,438,441; 2,438,432; 2,438,050; and, 2,438,101 all filed Aug. 22, 2003and a Canadian Patent Application, not yet assigned a serial number,filed Feb. 13, 2004. All of the applications listed above areincorporated herein in full by this reference to them.

FIELD OF THE INVENTION

[0002] This invention relates to a gas transfer apparatus and process,for example to support a biofilm in a liquid, as in a water orwastewater treatment process or apparatus.

BACKGROUND OF THE INVENTION

[0003] Currently, most wastewater treatment plants use an activatedsludge process, based on biological oxidation of organic contaminants ina suspended growth medium. Oxygen is supplied from air using bubble typeaerators. Efficiency of these systems is poor resulting in very highenergy use. Tank size is large since oxygen demand loadings are low. Theresult is high capital and operating cost.

[0004] A second type of established biological oxidation process usesbiofilms grown on a solid media. For example, the wastewater may becirculated to the top of the reactor and trickles down. Air is suppliedat the bottom. The rate of oxygen transfer is limited by the biofilmsurface area, and the operating cost is high because of wastewaterpumping requirements.

[0005] Recently, development work has been done on a membrane supportedbioreactor concept. For example, U.S. Pat. Nos. 4,181,604 and 4,746,435describe a process for treating wastewater by supplying oxygen from oneside of a gas-permeable membrane to micro-organisms growing on the otherside of the membrane. Hollow fibers with porous walls were used as themembrane. In U.S. Pat. No. 5,116,506, a gas permeable membrane divides areactor vessel into a liquid compartment and a gas compartment. Abiofilm is grown on the gas permeable membrane on the liquid side of themembrane. Oxygen and alternate gases pass through the membrane to thebacteria growing on the liquid side of the membrane.

SUMMARY OF THE INVENTION

[0006] It is an object of this invention to improve on the prior art. Itis another object of this invention to provide methods and apparatussuitable for treating water, for example industrial and municipalwastewater, using membrane supported bioreactor technology. It isanother object of this invention to provide a hollow fibre gas transfermembrane and module which is, for example, suitable for supporting abiofilm. These aspects and others are met by the invention described andclaimed herein. The following summary will introduce the reader tovarious aspects of the invention but is not intended to define theinvention which may reside in a combination or sub-combination ofvarious elements or steps found in the following summary or other partsof this document.

[0007] In one aspect, the invention provides a membrane and module witha reasonably high gas transfer rate and adequate surface area, foroxygen transfer, biofilm support or both, to allow a membrane supportedbiofilm reactor to provide an operating cost advantage over otherprocesses used in the art. The membrane and module may have an oxygentransfer efficiency (OTE) of over 50% or in the range of 50% to 70% ormore. The module may be made of non-porous or dense walled hollow fibremembranes to provide a large surface area while avoiding the tendency ofporous fibers to wet over time which results in a drastic drop in theiroxygen transfer rates.

[0008] In another aspect, the invention provides a very fine densehollow fibre made from poly methylpentene (PMP), which has a highselectivity and diffusion coefficient for oxygen. In particular, PMP hasa gas permeability of about 70,000 cc·mm/m²·24 hr·Bar in dense wall,non-wetting form. While this is significantly less than silicone, whichhas an extremely high gas permeability, PMP may be melt spun into ahollow fibre. The fiber can have an outside diameter of 500 microns orless or 100 microns or less. Use of such a small diameter fibre helpsreduce module cost as textile fine fibre technology can be used tocreate modules. A very large surface area can be provided to achievehigh OTE. The non-porous wall avoids wetting problems as describedabove.

[0009] In another aspect, the invention provides a fabric with a verylarge number of hollow fibres, for example of PMP, providing sufficientsurface area so that oxygen transfer does not become a limiting factorin controlling biological kinetics. The fabric may be made, for example,with the hollow fibres, optionally collected into units, woven as weftand an inert fibre as warp to minimize the damage to the transfer fibrewhile weaving. Other methods of preparing a fabric may also be used. Thefabric provides strength to the fine fibre to permit biofilm growth onits surface with minimal fibre breakage.

[0010] In another aspect, the invention provides a module built fromfabric sheets with very high packing density to permit good substratevelocities across the surface without recirculation of large volume ofliquid. The modules enable a supply of oxygen containing gas, such asair, to be supplied to the lumens of the hollow fibres without exposingthe lumens to the wastewater. Long fibre elements, for example between 1and 3 metres or between 1.5 and 2.5 metres are used and potted in themodule header to provide a low cost configuration.

[0011] In another aspect, a biofilm is grown on a fabric made from a gaspermeable hollow fibre, for example PMP dense wall hollow fibre. Oxygenbearing gas is introduced into the lumen of the fibre. Aerobic reactionstake place near the surface of the fibre, where the highest levels ofoxygen exists. These reactions include conversion of organic carboncompounds to carbon dioxide and water, and ammonia to nitrates. Thesurface of the biofilm is maintained under anoxic conditions such thatconversion of nitrates to nitrogen can take place. The result issimultaneous reduction of organic carbon, ammonia and total nitrogen.

[0012] In another aspect, the invention uses oxygen enrichment as ameans of dealing with peak flows. Need for such oxygen enrichment may bedetermined by on-line COD monitors, or set according to time of day for,for example, municipal applications where diurnal flow and strengthvariations are well known.

[0013] In another aspect, the invention uses the module and bioreactordesign to conduct other biological reactions on the surface of thefabric. An example is biological reduction of compounds such as nitratesin water using hydrogen gas supplied to the lumen of the hollow fibre.

[0014] In another aspect, the invention uses either air or enriched airto supply oxygen. Selection of enriched air and level of oxygen presentin such air may be determined by the wastewater strength.

[0015] In another aspect, the invention may be used to digest primaryand/or secondary sludge.

[0016] In another aspect, the fibres may have a small outside diameter,such as 100 μm or less, and substantial hollow area, for example 30% ormore or 40% or more, so as to have a thin wall. The fibres can be woven,knitted, stitched or otherwise made into a fabric. The use of finehollow fibres allows the thickness of the fibre wall to be low, forexample 20 μm or less, which is several times less than what would berequired to make a film handleable. The fine fibres may themselves bedifficult to handle on their own, but may be combined into units such asthreads or tows for handling which may include forming textile sheets.The fabric, having a large number of hollow fibres, provides sufficientsurface area for oxygen transfer capability such that air can be used asa feed gas without limiting the growth of the biofilm or otherbiological kinetics and with acceptable pressure loss due to air flowthrough the module.

[0017] In another aspect, plug flow or multistage continuous stirred orbatch tank reactors may be used to conduct biological reactions at thehighest possible substrate concentrations for a given feed. Thismaximizes mass transfer of organic carbon compounds and ammonia in thebiofilm, eliminating these processes as potential limitations toreaction rates. In multi-stage reactors, module designs with lowersurface areas for oxygen transfer to biofilm surface area ratios may beused in downstream stages. The total surface area for oxygen transfer,for example per unit of tank volume or flow rate of feed, may increaseor decrease in the downstream reactor since the lower ratio may resultfrom an increase in biofilm surface area rather than a decrease insurface area for oxygen transfer.

[0018] In another aspect, the invention provides a membrane supportedbatch biofilm reactor (MSBBR). The reactor includes one or more membranemodules which are fed an oxygen containing gas and support a biofilmlayer. The modules are located inside of a tank that is cyclicallyfilled and drained to provide a batch treatment process. In anembodiment, the modules are made of a hollow fibre fabric and are usedto reduce the COD, ammonia, total nitrogen and suspended solids in anindustrial wastewater to concentrations suitable for discharge into amunicipal sewer system or for direct discharge to a receiving stream. Inanother embodiment, the modules are used to reduce COD, ammonia, totalnitrogen and suspended solids in a municipal wastewater stream fordirect discharge to a receiving stream. In another embodiment, themodules are used to reduce COD, ammonia, total nitrogen and suspendedsolids in a septic tank to reduce the size of the septic field or to usesimpler, lower cost disposal techniques or for direct discharge to areceiving stream.

[0019] In another aspect, the invention provides one or more methods ofcontrolling the growth or thickness of a biofilm layer growing on themodules. Some method(s) involve applying one or more substances to thebiofilm from the tank side while the tank is drained of feed. Thesesubstances may include gases, such as ozone or chlorine, or liquid suchas heated water or basic or acidic solutions. During the application ofthe control substance, conditions in the biofilm may be cycled fromaerobic to anaerobic by turning the supply of oxygen to the inside ofthe module on and off. The biofilm may also be starved prior to theapplication of the control substance by removing the feed water,replacing the feed water with clean water or replacing the feed waterwith feed at a loading of 0.1 kg COD per kg MLSS per day or less. Afterthe application of the control substance, mechanical biofilm controlmethods may also be used on the weakened biofilm.

[0020] In another aspect, this invention uses scouring air provided onthe outsides of the fibres as a means of controlling the biofilmthickness to an optimum level. Air may be used as a means of controllingthe biofilm thickness to a desired level. Treatment with acid, alkali,oxidant, or enzyme, or anaerobic treatment may be used periodicallyprior to air scouring to weaken the biofilm and to improve the efficacyof air in completely or partially removing the biofilm. Other methods ofbiofilm control include in-situ digestion, periodic ozonation followedby digestion, periodic alkali or acid treatment followed by digestion,periodic enzyme treatment followed by digestion, and use of a higherlife form, such as worms, to digest the biofilm periodically. To speedup the biological digestion reactions, the air supplied to inside of themodule may be preheated to raise the temperature of the bioreactor.

[0021] In another aspect, the invention provides a tow of hollow fibers,for example with an outside diameter (OD) of 500 microns or less or 100microns or less. To facilitate building modules with minimal reductionin the effective surface area of the fibres, the fibres are processed orused as tows over a significant portion, for example one half or more,of their length. Modules may be made directly from the tows withoutfirst making a fabric. The tows may also be made into open fabrics tofacilitate potting, for example along the edges of the fabric, whileleaving significant portions of the fibres as tows, for example aportion between the edges of the fabric. The modules made from tows maybe potted at both ends, or potted at one end only with the other endleft unpotted with fibre ends open to permit exhaust gas to escape. Asingle header module may have lower cost than a double header module. Asingle header module may be inserted in a vertical configuration withthe header at the bottom and the fibres floating upwards. Such a modulemay be aerated from outside the module to remove accumulations of trashand solids. Feed may also be screened, for example through a 0.5 mmscreen, to reduce trash in the feed before it enters the reactor. Wherethe tow module is used in a downstream stage of a multi-stage reactor,the upstream stage may also reduce the amount of trash fed to the towmodule reactor.

[0022] In another aspect, reactors for treating wastewaters of differentstrength are provided with modules having different ratios of surfacearea for gas transfer to surface area of the attached biofilm. Thesurface area for gas transfer is the area of the outer surface of themodule that is in contact with the supported biofilm. The surface areaof the biofilm is the area of the outer surface of the biofilm thatcontacts the wastewater. Is some cases, the surface area of the biofilmdepends on the thickness of the biofilm which, for calculations or forcomparing modules, may be the actual thickness or time average ofthicknesses of a biofilm in a rector or a nominal or design thickness oraverage thickness, for example 250 microns. A reactor for treatingwastewater with a COD of over 1000 mg/L may have a module with a surfacearea for gas transfer to surface area of attached biofilm ratio of morethan 1, more than 1.6, or between 1.6 and 10. A reactor for treatingwastewater with a COD of less than 1000 mg/L may have a module with asurface area for gas transfer to surface area of attached biofilm ratioof less than 2.5 or between 0.2 and 2.5. A reactor for treatingwastewater with a COD of less than 300 mg/L may have a module with asurface area for gas transfer to surface area of attached biofilm ratioless than 1 or between 0.1 and 10. In a multi-stage process, two or morereactors may be connected in series with the outlet of an upstreamreactor connected to the inlet of a downstream reactor. The COD of thewastewater to be treated decreases through each reactor and the surfacearea for gas transfer to surface area of attached biofilm ratio formodules in a downstream reactor is less than for modules in an upstreamreactor.

[0023] Other aspects of the invention are described in the claims or inthe following drawings or description.

BRIEF DESCRIPTION OF THE DRAWINGS

[0024] Embodiments of the invention will be described below withreference to the following figures.

[0025]FIG. 1 is a picture of a group of hollow fibres.

[0026]FIG. 1a is a cross-section of a hollow fiber.

[0027]FIG. 1b shows a group of hollow fibers and inert fibers collectedinto a unit.

[0028]FIGS. 2a through 2 d and 2 e show slot arrangements and aspinneret for melt spinning fibers.

[0029]FIGS. 3a and 3 b show a plan view and cross-section of a wovenfabric respectively.

[0030]FIG. 3c shows steps in weaving a fabric.

[0031]FIGS. 3d shows a warp knitted fabric.

[0032]FIG. 4a shows a sheet of hollow fibres with a central portion ofthe sheet having the fibres in tows. FIG. 4b shows details of a part ofthe sheet of FIG. 4a.

[0033]FIG. 5 is a cross-section of a loose tow module.

[0034]FIG. 6 shows a top view of a module having sheets of fibres.

[0035]FIG. 7 is a partial section, in elevation view, of the module ofFIG. 6.

[0036]FIG. 8 is a cross-section of another part of the module of FIG. 6in plan view.

[0037]FIG. 9 is an elevation view of a module according to FIGS. 6 and7.

[0038]FIGS. 10a, 10 b and 10 c are elevation, plan and partial sectionviews of another module having sheets of fibres.

[0039]FIGS. 11 and 12 are plan and elevation views of a tank havingcassettes of modules of sheets of hollow fibres.

[0040]FIG. 13 is a drawing of the details of a tensioning mechanism inthe apparatus of FIGS. 11 and 12.

[0041]FIG. 14 is an elevation view of the mechanism of FIG. 13.

[0042]FIGS. 15 and 16 are schematic elevation drawings of reactors.

[0043]FIGS. 17 and 18 are schematic drawings of other reactors.

[0044]FIG. 19a is a bench scale batch reactor using a tow module.

[0045]FIG. 19b is a photograph of a biofilm on a tow of fibres growingin the reactor of FIG. 19a taken through a microscope.

[0046]FIG. 20 is a schematic elevation drawing of a septic tank modifiedto use a supported biofilm module.

[0047] FIGS. 21 to 31 are results of tests conducted with various samplemodules or reactors.

DESCRIPTION OF EMBODIMENTS

[0048] 1.0 Module Elements

[0049] 1.1 Fiber

[0050]FIGS. 1 and 1a show a Poly (4-methylpentene-1) (PMP) fiber 10 thatis hollow inside but non-porous with dense walls. In a group of fibers10, the fibers 10 may have various diameters, and may be fine fibershaving outside diameters of less than 500 microns or less than 100microns, for example, between 30 and 100 microns, or between 50 and 60microns. The hollow fibres 10 shown are non-porous, or dense walled, andwater does not flow through the fiber walls by advective flow. However,oxygen or other gases may permeate or travel through the fiber walls,for example by molecular diffusion or dissolution-diffusion.

[0051] The hollow fiber 10 can be prepared by melt spinning, alternatelycalled melt extrusion. In melt spinning a polymer granulate, for exampleof PMP, is fed to the hopper of an extruder. The polymer granulate isheated and melted in the extruder and continuously extruded to aspinning head under a pressure of several tens of bars. The spinninghead consists of a heated in-line filter and spinneret. The spinneret isessentially a steel plate with thin arc shaped slots in circulararrangements. Examples of suitable slot arrangements for the formationof a hollow fiber are shown in FIGS. 2a to 2 d. As shown in FIG. 2e, thespinneret may have multiple groups of slots so that many fibers, 8 inthe spinneret shown, can be extruded simultaneously. The molten polymeris extruded through the spinneret, leaves the slots and closes into ahollow fiber in a cooling zone. The gaps caused by the segment dividersallow air into the fibre to prevent collapse before the fibre sectionsfuse to form the annulus. In the cooling zone, the polymer fiber form issolidified and cooled by a controlled cross flow of air and the end iscollected on a take up winder. Suitable fibers 10 may also be formed byother melt spinning methods. For example, in pipe in hole spinning thepolymer is melted and drawn through an annular spinneret while passing agas into the lumen of the extruded fibers through another hole in thespinneret to prevent fiber collapse. Methods other than melt spinningmay also be used.

[0052] Referring to FIG. 1a, in the illustrated embodiment, a meltspinning method is used to make fibres 10 with an outside diameter 12 of100 μm or less. The hollow area (or area of the lumen 14) of the fibremay be more than 10% or more than 30% or 40% of the cross-sectional areaof the fibre. The hollow area is typically less than 60% or 50% of thecross-sectional area of the fiber. For example, a polymethyl pentenefibre may be made having an outside diameter 12 of between about 50 to60 μm and an inside diameter 16 of 30 μm or more, resulting in a wallthickness 18 of 10 μm or less and a gas permeability of over 30,000cc·mm/m²·24 hr·Bar or more.

[0053] In the embodiment illustrated in FIG. 1, the textile PMP fibre 10has about a 45 micron outside diameter 12 and about a 15 to 30 microninside diameter 16. The fibre 10 was melt extruded using MX-001 orMX-002 PMP, produced by Mitsui Petrochemical of Japan and sold under thename TPX, as the raw polymer through a segmented spinneret as describedabove. This fiber 10 is used in the embodiments and examples describedin this document, although other fibers 10 may also be used.

[0054] 1.2 Fiber Aggregates (e.g. Tows)

[0055] Referring to FIG. 1b, the hollow fibers 10 may be combined intofiber units 19 for handling. The fibre units 19 may be individual fibres10, tows 20, for example, of 1 to 200 or 16 to 96 fibres 10 each, eithertwisted or untwisted (FIG. 1b), threads, yarns, tubular, flat or cordagebraids, or other units 19 for handling. Tows 20 are made by re-windingfibers from multiple take up spools in combination on to a second spool.Stronger inert fibres 22, such as yarns of PE or PP, may be included ina tow 20 or other unit 19. The fibres 10 may be curled for use in theunits 19. Curled fibres 10 can be made by winding them onto a bobbin atvarying tensions.

[0056]1.3 Sheet Structures

[0057] The fibers 10 and/or fiber units 19 can be provided in the formof sheets 26. In FIGS. 3a and 3 b, the fibres 10 are woven as fibreunits 19 into a basic two-dimensional structure or fabric sheet 26. Inthe embodiment illustrated, the units 19 run across the sheet, meaningperpendicular to the direction in which the sheet 26 advances out of aloom. Inert fibres 22 run along the length of the sheet 26 to providesupport to the fibre units 19. FIG. 3c illustrates steps involved in aweaving process. The fiber units 19 are carried on a shuttle through 2groups of inert fibers 22 that are alternately raised or lowered aftereach pass of the shuttle. Other weaving or fabric making methods mayalso be used. Unit 19 type, unit 19 bundle size, spacing between units19 and percent of fibre in each direction can all be tailored to meetthe mechanical or biochemical requirements of each unique application.

[0058] In more detail, the fibre units 19 provide a support surface forthe growth of a biofilm 30. The number of hollow fibre units 19, and thenumber of fibres 10 per unit 19, may be adjusted to provide a desiredsurface area for O₂ transfer compared to surface area of biofilm 30 orto the planar surface area of the fabric sheet 26. The planar surfacearea of the sheet 26 is simply the sheet length multiplied by its width,multiplied by two (since the sheet has two sides). The surface area ofthe biofilm 30 is the total area of the biofilm 30 exposed to the liquidin the reactor, which may be generally the same as the planar area ofthe sheet 26 for a substantially two dimensional sheet configuration.

[0059] The surface area for O₂ transfer is the total area of the hollowfibres 10 in the sheet exposed to the biofilm. This is approximatelyequal to the product of the effective diameter and length of the fibre10, multiplied by the number of the fibres 10 in the sheet 26. Theeffective diameter for diffusion is a logarithmic average of thediameters of the fibre to account for the effect of the wall thickness.The inert fibres 22 crossing the hollow fibres 10 in the sheet 26, andcontact between fibres 10, may interfere with oxygen transfer in someembodiments, for example a tightly woven fabric, but the interference isgenerally small and is ignored in surface area for oxygen transfercalculations.

[0060] Although the surface area of the biofilm 30 is generally the sameas the planar area of the sheet, it may be slightly larger for veryrough or open fabrics or fabrics having more dispersed fiber units 19.Varying fabric roughnesses may also be used to affect the thickness ofthe biofilm 30 or how readily the biofilm 30 can be reduced orcontrolled. High ratios of O₂ transfer surface area to biofilm area (SAO₂/SA biofilm) may be obtained, in the range of, for example, 6 to 10 ormore. However, for treating feed water with a high concentration of COD,for example, 300 mg/L CODs or more, lower SA O₂/SA biofilm ratios, forexample, between 1.6 and 10 are sufficient, and may be preferred toreduce module cost. An SA O₂/SA biofilm ratio in the range of about 2 to8, or about 4 to 6, can provide satisfactory results in many treatmentapplications.

[0061] The surface area of the biofilm 30 can also be larger than theplanar area of the sheet 26 by providing a loose arrangement of fibres10 and controlling the thickness of the biofilm 30 to a sufficientlythin layer so that the biofilm 30 on adjacent parallel fibres does notform a continuous layer. A sheet 26 with a rough or textured surface,the height of the surface undulations being in the range of the desiredbiofilm thickness, may also be desirable since it may facilitate biofilmcontrol. Desired biofilm thickness may be 200 to 1,000 microns.

[0062] Provided that oxygen transfer through the module 40 does notlimit reactions in the biofilm 30, the rate of COD reduction in thewastewater is roughly proportional to the concentration of COD in thewastewater. However, for oxygen transfer to not be a limiting factor,more oxygen is required to flow through the module 40 to support abiomass of the same surface area as wastewater COD concentrationsincrease. More oxygen can be provided by increasing the size or speed ofoperation of a blower. However, large head losses, for example 10 psi ormore, may result due to resistance to oxygen flow through the fibrelumens 14. Head loss may be kept below 10 psi, or in the range of 6 to 9psi, by choosing a fabric type and number of fibres that producessufficient total lumen area for a given biofilm outer surface area.

[0063] Also, the inventors have observed that biofilms growing inwastewater with high concentrations of COD, for example 1000 mg/L CODsor more or 2000 mg/L CODs or more, are more resilient and tend to growto undesirable thickness of a few mm or more, faster than biofilmsgrowing in wastewater with lower COD concentrations. Thus, biofilmsgrowing in high COD wastewater require more strenuous biofilm controlmethods which in turn make a stronger fabric desirable.

[0064] The various issues discussed above make it preferable for fabricsto be used in high COD wastewater that have more fibres, and optionallymore surface roughness, for the same overall planar area of a sheet orouter surface area of supported biofilm than for fabrics used to treatlower COD wastewater. This can be achieved by choice of method used tocreate the fabric and choice of thread or fabric unit count or tightnessof the fabric. Multi-stage reactors may also be used. In a multi-stagereactor, an upstream reactor treats the feed at its highest CODconcentration and is fitted with modules having dense fabrics with largenumbers of fibres. A downstream reactor receives partially treatedwastewater with a lower COD and is fitted with modules having a lessdense fabric with fewer fibres for the same sheet or biofilm outersurface area. The less dense fabric is more economical since it has lessfibres and may have a higher area of biofilm for a sheet of the sameplanar surface area.

[0065] The fabric sheets 26 may also be made by other methods such asbraiding, stitching or knitting, such as warp knitting. Warp knitting isdesirable, for example, when small units 19 or tows or even individualstrands of fine fiber 10 are used. The fabric sheets 26 may bepatterned, as in pattern knitting, if desired, to provide areas withfewer fibers or holes to enhance flow through the sheets 26.

[0066] In warp knitting, the fabric sheet 26, as shown in FIG. 3d,contains interlaced loops of ‘knitted stitches’. The column of stitchesbeing formed on one needle make a fringe. The fringes in the lengthdirection (‘warp’) of the fabric can be made by relatively inexpensive,commodity yarns, e.g. PET, PP, etc., as the inert fibres 22. The inertfibres 22 can withstand the stress and strain of processing and use. Thefabric sheet 26 is generally strong and stiff in the warp (length)direction and elastic in the weft (cross) direction. The weft is aperpendicular yarn system, which is laid across the fringes and fixed bystitches (loops) of the warp fibres 22. The weft doesn't take part inthe fabric (loop) formation, therefore the weft fibre units 19 can beprocessed very tenderly, being subjected to less stress and strain thanthe warp. Accordingly, preparing the sheet 26 with units 19 as the weftcan minimize risk of damage to the fibres 10 during manufacturing thesheet 26. The weft is usually a parallel layer or band of yarns beingmoved crosswise to the fringes (warp) during knitting. The fabric sheetwidth can be about to 2-3 m.

[0067] In the embodiment of FIGS. 4a and 4 b, the sheets 26 areconstructed of an open fabric made by weaving tows 20 through theshuttle of a loom and crossing the tows 20 with an inert fibre 22 onlyalong the edges of the fabric 26. The fabric shown is approximately 1.3m wide, that is it has active fibres 10 of about 1.3 m long, and hasinert fibers 22 woven perpendicularly to the tows 20 in a strip of about2 cm along the edges. As shown in FIG. 4b, the fibers 10 in each tow 20disperse beyond the strips so that the tows 20 remain unrestrained andpartially open between the strips. The resulting roll of 1.3 m widefabric is cut into sections of about 20-200 cm, or 30-60 cm, width tomake individual sheets 26. In FIG. 4b, the number of fibers 10 in eachtow 20 is small for clarity but the tows 20 may each have, for example,between 1 and 200, for example 16, 48 or 96 fibers 10.

[0068] 1.4 Modules

[0069] 1.4.1 Loose Tow Module

[0070] In accordance with the present invention, multiple fiber units19, including fibers 10, tows 20 or sheets 26, can be grouped togetherto form membrane modules 40. FIG. 5 shows a module 40, which may becalled a tow or loose tow module, with fibres 10 arranged and potted intows 20 of fibres. The tows 20 are made of a loose collection of aplurality of fibres 10, for example between 1 and 200 or 16 to 96 fibres10. The fibres 10 may be lightly twisted together or left untwisted. Thefibres 10 may be curled, crimped or undulating to provide threedimensional structure to the each potted row. Curling may be achieved byre-winding the fibres 10 onto a bobbin while varying the tension on thefibres. The individual fibres 10 remain separable from each other in thetow 20. Such a tow 20, when coated with a thin biofilm, for example ofless than 1 mm thickness, may provide a ratio of gas transfer areathrough the fibre walls to biofilm outer surface area(SA_(oxygen)/SA_(biofilm)) of less than 2.5, less than 1 or between 0.1or 0.2 and 1. Inert fibres 22 may be added to the tow to strengthen itif required. Each tow 20 is potted into a plug of resin 32 so that itsends 34 are open at one face of the resin 32. The plug of resin 32 isglued into a plastic header enclosure 35 having a port 36 which forms aheader 44 connecting the port 36 to the open ends 34 of the fibers 10through a cavity 37. There are two headers 44, one associated with eachend of the fibres 10, although modules 40 with only an inlet header 44may also be made. With two headers 44, air or other gases may be inputinto one header 44, flow through the fibres 10 and exhaust from thesecond header 44. Tows are potted in a resin 32, such as polyurethane,and the potted ends are cut to expose the fibre lumen. Alternately, afugitive potting material may be used to block off fibre ends, asdescribed in U.S. Pat. No. 6,592,759, or other potting methods may beused. In FIG. 5, the number of tows 20 and the number of fibers 10 pertow 20 are both small for clarity in the drawing and may be much largerin practice.

[0071] 1.4.2 Sheet Module

[0072] A module 40 can also be constructed of a bundle or stack ofsheets 26. The sheets 26 may have perpendicular inert fibers presentacross the entire width of sheet 26 as in FIG. 3a or only across aportion of the width of the sheet 26, for example at the ends as in FIG.4. Raw material for the sheets 26 may be rolled onto a fabric roll. Forexample, where the sheets 26 are prepared by weaving, the material isrolled on to a take up roll at the end of a loom as material isproduced. The fiber units 19 may extend across the roll while the inertfibers spiral around the roll. With the fibers oriented in this way,individual sheets 26 may be cut from the roll by rolling out a length ofmaterial from the roll and cutting it off with a hot knife or heatcutter. The heat cutter melts through the fiber units 19 and inertfibers and bonds them together to protect the fabric edge fromdisintegrating or fraying. Since the heat cutter melts a strip of fiberson either side of the cut line, for example a strip about 5 mm wide, thefibers remaining on the roll are similarly melted together to produce astable edge. After a sheet 26 has been cut from the roll, the other twoends of the sheet, meaning the edges of the sheets 26 at right angles tothe heat cut edges, are cut to open the lumens of the fiber units 19. Tominimize distortion or collapse of the ends of the fibers 10 under thecutting pressure, the area to be cut is first reinforced, for example byimpregrating it with polyurethane to provide a reinforcing coatingaround the fibers 10 or fiber units. The cut across the fiber units 19is then made with a sharp cutter, for example a razor edge cutter. Thecutter is preferably kept very sharp, for example by changing bladesregularly, to minimize distortions of the ends of the fibers 10. Othercutting machines or tools used in the garment and textile industries mayalso be used.

[0073] The end or ends of single or multiple sheets 26 can be pottedinto a header to provide one or more ports 36 in communication with thelumens of the fibers 10. To pot one or more sheets 26, sheets 26 are cutfrom a roll as described above. A plastic spacer strip is attached, forexample with glue or adhesive transfer tape, on one or both sides of thesheet 26, at the end of the sheet 26 parallel to but offset from therazor cut line across the fiber units 19. For potting multiple sheets26, the sheets 26 with spacer strips attached are laid on top of eachother and attached together, for example by glue or adhesive transfertape, between adjacent spacing strips or between the spacing strip ofone sheet 26 and a second sheet 26. The strips space adjacent sheets 26but also form a barrier between a potting material to be applied laterand the cavity of the header containing the ends of the fibers 10. Theends of the sheet 26 or stack of sheets 26 is fitted into an elongatedheader cavity that may be made, for example, by injection molding.Spacing and sealing to the header walls is maintained with aself-adhesive closed cell neoprene gasket strip attached to each of thelong header walls. Any openings in the header cavity left by the spacerstrips may be covered with hot melt glue. Final sealing of the header isachieved by pouring a layer of potting material, for example atwo-component polyurethane compound, over the spacer strips. The layermay be about 45 mm thick and extend between the insides of walls of theheader. If there are multiple sheets, care is taken to force or ensureflow of the potting material, as completely and evenly as practicable,between the sheets 26. After the potting material hardens, a seal isformed between the outsides of the fibers 10 and the walls of the headerbut the ends of the fibers 10 remain in communication with a cavitywithin the header.

[0074] FIGS. 6 to 9 show a module 40 in which a set of parallel sheets26 are potted with gaps 42 between them in a header 44. Two headers 44may be used as shown when a bleed of exhaust air is desired. One header44 may also be used with exhaust bled through opposed open ends of thefibres 10 or with the other ends of the fibres 10 closed for dead endoperation. The gap 42 may be between 2 mm and 10 mm thick, or between 3mm and 15 mm. The chosen gap 42 may depend on the water to be treated orthe choice of method to control biofilm thickness. For example, a module40 of tensioned sheets 26 may have a gap 42 of 6 mm when used with airscouring to control biofilm 30 thickness. Tension may be provided bymounting the headers 44 to a rigid structure, which may include parts ofa tank, with one or both headers 44 movable relative to the structure.Alternately, the headers 44 may be attached to part of a frame held atan adjustable distance apart. The sheets of fabric 26 are potted andseparated in the headers 44 by various potting materials 46 such as oneor more of polyurethane, hot melt glue, adhesive strips, plastic spacingstrips or epoxy. The spacing between adjacent sheets 26, or gaps 42,provides space for scouring air and substrate flow through the module40. A large sheet of the fabric 26 may also be rolled or folded toproduce a module 40 rather than using separate sheets. The length of themodule 40 is a compromise between OTE and pressure drop and may rangefrom 1 m to 5 m or between 1 m and 3 m.

[0075] Referring to FIG. 8, to make the module 40 a sheet 26 of fibres10 is laid onto strips 50 (one on each end) of adhesive located to crossthe ends of the fibres 10. Additional strips 50 of adhesive and spacingstrips 52 are placed over the sheet 26, followed by additional strips 50of adhesive and an additional sheet of fabric 26. These steps arerepeated as appropriate for the number of sheets 26 desired. Theresulting assembly is then sealed into the header enclosures 35 of apair of opposed headers 44 such that the lumens 14 of the fibres 10 arein communication with ports 36 in the headers 44 through cavities 37.The ends of the fibres 10 are cut before potting to open them, forexample as described above. Additional glue or potting resin 41 mayoptionally be poured into the header enclosure 35 to further seal thefibers 10 to the header enclosure 35. Alternately, sheets 26 may beseparately glued to spacing strips at their edges and inserted into aheader cavity and additional glue or potting resin 41 placed around thisassembly to seal it to the header enclosure 35. Further alternately, thefirst assembly method described above may be used.

[0076]FIG. 9 shows a picture of a module 40 assembled as generallydescribed above. The headers 44 are about 2 meters apart. Additionalspacers 33 are used mid way between the headers to better preserve thesheet 26 separation. A thin steel rod 45 is attached to the edges of thefabric sheet 26 in the right half of the module to address the foldingwhich can be seen in the left half of the module. The module 40 has aratio of SA oxygen/SA biofilm of about 5.

[0077] Another embodiment of a module 40 can be seen in FIGS. 10a to 10c. The module 40 has a single sheet 26 with hollow fibre units 19 andinert fibres 22. The hollow fibre units 19 extend between headers 44 ateither end of the sheet 26. The width 62 of the headers 44 is such thatstacking multiple modules 40 adjacent each other with the headers 44 ofadjacent modules 40 abutting each other provides the desired spacingbetween the adjacent sheets 26. The header enclosures 35 of this module40 are clear allowing the cavity 37 to be seen. To pot the sheet 26, theheader enclosure 35, which is a folded over plastic strip, is forcedopen and a sheet 26 is inserted. The header enclosure 35 springs closedon the sheet 26. Tubes which function as ports 36 are inserted into theends of the header enclosures. Potting resin 31 is laid along the jointbetween the sheet 26 and the header enclosure 35, between the ports 36and the header enclosure 35 and all other openings to seal the cavity37.

[0078] Referring again to FIG. 4, another module, which may be called atow or tow sheet module, can be made of open sheets 26 of tows 20 cutalong the woven edges to open the ends of the fibres 10 and potted witha 0 to 10 mm space between them into one or a pair of opposed headers.Depending on the potting method used, which may include potting methodsdescribed above, the fibres 10 may be cut open either before or afterthey are inserted into the potting resin. 1 to 100 or 8-20 sheets may bepotted into a pair of headers to produce a module. Modules made in thisway using the fibers of FIG. 1 had SA_(oxygen)/SA_(biofilm) ratios ofbetween 1:2.5 (0.4) and 1/11 (0.1) with a biofilm thickness of 250microns.

[0079] 1.5 Cassettes/Reactors

[0080] In general, a plurality of modules can be grouped together toform a cassette, and one or more modules or one or more cassettes can beplaced in a tank as part of a reactor. Referring to FIGS. 11 and 12, themodules 40 of a cassette 110 are mounted in a tank 112 of a pilotreactor for treating 1 cubic meter per day of industrial wastewaterhaving a COD of over 1,000 mg/L, typically 7,000 mg/L. The feed istreated by either batch or continuous process to reduce its CODconcentration to 300 mg/L as required for discharge into the municipalsewer that it outlets to. The tank 112 has a fill volume of 1.8 m³.Fifteen modules 40 are provided in the tank 112, each module 114containing six sheets 26 of 3.6 m² surface area of a woven fabric of PMPfibers units 19, woven as tows 20. The fibres 10 are 1.8 m long andextend between an inlet header 116 and outlet header 122 of the modules40. Total number of PMP tows per sheet is 1968, and fibres per sheet are94464, there being 48 fibers per tow and a two packing of 50 threads perinch in the sheet 26. Also, polyester yarn is woven perpendicular to thePMP fibre, and the total number of yarns per module is 1912. Airpressure drop in the fibre lumen is in the range of 5 to 10 psi. Totalbiofilm area per module is 17 m², and oxygen transfer area is about 5.1times the biofilm area.

[0081] The modules in the embodiment illustrated are mounted in such away that the tension of the sheets 26 extending between the headers 116,122 can be adjusted. The cassette provides a rigid structure 150, whichcan include elements of the tank 112 or elements of a cassettesub-frame, adjacent the modules 40, and one or both of the headers 116,122 are movable relative to the rigid structure 150.

[0082] In the embodiment illustrated, the rigid structure 150 comprisesa pair of side plates 152 that extend along the distal side surfaces ofthe outermost modules 40 of the stack of modules 40. As best seen inFIGS. 13 and 14, the modules 40 are attached to the side plate 152 bymeans of a mounting bracket 154 extending transversely between the sideplates 152 at either end of the modules 40. The mounting brackets 154are provided with grooves 156 shaped to receive T-shaped tongues 158extending from surfaces of the headers 116, 122, opposite the sheets 26.The module 40 can be secured to the mounting brackets 154 by sliding thetongues 158 of the headers 116 and 122 into the grooves 156 of thebrackets 154. The mounting brackets 154 can be secured to the side plate152 by, for example, a bolt 160 passing through an aperture 162 engagingthe plate 152 and a threaded hole 164 in an edge surface of the bracket154.

[0083] The aperture 162 can be slot-shaped, so that the bracket 154 withthe attached header 116, 122 can be shifted horizontally to increase ordecrease the tension of the sheets 26. An eccentrically mounted cammember 166 can be provided between the head of the bolt 160 and theplate 152, with an outer diameter surface in engagement with an abutmentsurface 168 fixed to the plate 152. Rotating the cam member 166 canforce the opposed brackets 154 further apart or allow them to drawcloser together, thereby adjusting the tension of the sheets 26 in themodules 40.

[0084] The tension adjustment mechanism can be provided on only one endor on both ends of the modules 40, and can be modified to provideindividual tension adjustment for each module 40 or for sub-groups ofmodules 40. Other mounting methods may also be used to allow modules 40to be removed or tensioned.

[0085] In another embodiment of the invention, the elements or modulesare stacked in a vertical configuration. Flow of scouring air fromoutside the modules or of water in the tank may be from top to bottom orbottom to top. This minimizes the capital required for scouring air andthe operating cost of air.

[0086] 2.0 Operation/Applications

[0087] The fiber units 19 having one or more fibers 10 can be used asmembranes to support biofilm in a reactor. In general, gas containingoxygen flows into at least one of the headers 44 of a module 40. Themodule 40 may be operated in a dead end mode, with no outlet other thanthrough the fibres. Alternately, the module may be operated in a crossflow manner with gas entering through one header 44, flowing through thefibers 10, then exiting from the other header 44. The oxygen content andflow rate of the gas may be set to produce an oxygen transfer thatprovides aerobic conditions near the outer surface of the fibers 10,where the level of oxygen is highest. Aerobic reactions occur in thisarea, including conversion of organic compounds to carbon dioxide andwater, and ammonia to nitrates. The biofilm may be maintained underanoxic conditions on its outer surface or near the substrate beingtreated and conversion of nitrogen to nitrates can take place. In thisway, multiple and simultaneous reactions, including carbon basedorganics, ammonia and total nitrogen reduction, may be performed in thebiofilm.

[0088] An example reactor 80 is shown in FIG. 15. FIG. 15 provides anear plug flow. The reactor 80 has a tank 82, a feed inlet 84 to thetank 82, an effluent outlet 86 from the tank 82, a flow path 88 betweenthe feed inlet 84 and the effluent outlet 86, and a plurality of fiberunits 19 in the form of modules 40 in the tank 82. Each module 40 canhave one or more sheets 26 extending from one or more headers 44. Theplurality of modules 40 can be provided as part of one or more cassettes110.

[0089] The sheets 26 and modules 40 are sized to fit the tank 82 andfill a substantial part of its volume. The sheets 26 may be custom madeto provide efficient use of the available space in the tank 82. Thesheets 26 are preferably arranged in the tank 82 in a number of rows,one such row being shown in FIG. 15. The sheets 26 may range from 0.25to 2 mm in thickness and adjacent sheets 26 are placed in the tank 82side by side at a distance of 2 to 15 mm to allow for biofilm growth andwastewater flow between adjacent sheets 26.

[0090] The tank 82 is longer than it is deep and may have a generallyhorizontal flow path 88 with minimal mixing. This is achieved by leavingsome space near the ends (ie. near the inlet 84 and outlet 86) of thetank 82 for vertical movement of water and leaving minimal free space atthe top, bottom and sides of the tank 82. A baffle 90 may also be placedupstream of the effluent outlet 86 to force the flow path 88 to go underit. A sludge outlet 92 is provided to remove excess sludge.

[0091] The flow path 88 is generally straight over a substantial portionof the tank 82 between the feed inlet 84 and effluent outlet 86. Eachmodule 40 is held in the tank 82 by its headers 44 attached to a frame(not shown for clarity) which restrains each module 40 in positions inthe reactor 80 whereby the sheets 26 of each module 40 are generallyparallel to the flow path 88. Preferably, a plurality of sheets 26 arespaced in series along the flow path 88 so that the reactor 80 will morenearly have plug flow characteristics. Wastewater to be treated may bepartially recycled from the effluent outlet 86 to the feed inlet 84.Such a recycle can increase the rate of gas transfer by increasing thevelocity of wastewater along the flow path 88, but it is preferred ifthe recycle ratio is small so as to not provide more nearly mixed flowcharacteristics in the reactor 80.

[0092] Oxygen containing gas is provided to each module 40 through itsinlet conduit 216 connected to an inlet manifold 94 located above thewater to be treated. With the inlet manifold 94 located above the water,a leak in any module 40 will not admit water into the manifold nor anyother module 40. Gas leaves each module 40 through its outlet conduit218 which is connected to an exhaust manifold 95. Although it is notstrictly necessary to collect the gases leaving each module 40, it doesprovide some advantages. For example, the gas in the exhaust manifold 95may have become rich in volatile organic compounds which may createodour or health problems within a building containing the reactor 80.These gases are preferably treated further or at least vented outside ofthe building.

[0093] Oxygen diffuses or permeates through the fibers 10. The amount ofoxygen so diffused or permeated may be such that an aerobic biofilm iscultured adjacent the sheets 26, an anoxic biofilm is cultivatedadjacent the aerobic biofilm and the wastewater to be treated ismaintained in an anaerobic state. Such a biofilm provides forsimultaneous nitrification and denitrification. A source of agitation 98is operated from time to time to agitate the sheets 26 to releaseaccumulated biofilm. A suitable source of agitation is a series ofcoarse bubble aerators which do not provide sufficient oxygen to thewater to be treated to make it non-anaerobic.

[0094]FIG. 16 shows a second reactor 80 having a tank 82, a feed inlet84, an effluent outlet 86, a flow path 88 and a plurality of modules 40.Frames (not shown) hold each module 40 in a position whereby the sheets26 of each module 40 are generally parallel to the flow path 88.

[0095] The sheets 26 are sized to fit the tank 82 and fill a substantialamount of its volume. The sheets 26 may be custom made to provideefficient use of the available space in the tank 182. The sheets 26 mayrange from 0.25 to 2 mm in thickness and are placed side by side at adistance of 2 to 15 mm to allow for biofilm growth and wastewater flowbetween adjacent sheets 26.

[0096] The tank 82 is deeper than it is long to encourage a straight andgenerally vertical flow path 88 over a substantial portion of the tank82 with minimal mixing. This is done by leaving minimal space near theends and sides of the tank 82 but a substantial amount of space near thetop and bottom of the tank 82. Water to be treated may be partiallyrecycled from the effluent outlet 86 to the feed inlet 84 but it ispreferred that the recycle rate be small if a recycle is used.

[0097] Oxygen containing gas is provided to each module 40 through itsinlet conduit 216 connected to a manifold 94. The manifold 94 mayalternately be located above the water to be treated so that a leak inany module 40 will not admit water into the manifold 94 nor any othermodule 40. Outlet conduits 218 are connected to an outlet manifold 95which may alternately be located above the surface of the water to betreated.

[0098] Alternatively, gas flow through the module 40 is produced byapplying a suction to the outlet conduits 218. The inlet conduits 216are placed in fluid communication with the atmosphere. By this method,the rate of gas diffusion across the membrane is slightly reduced, butthe exhaust from the blower may be connected to further apparatus forprocessing the exhaust gases.

[0099] Oxygen diffuses or permeates through the membranes 120 preferablysuch that an aerobic biofilm is cultured adjacent the sheets 26, ananoxic biofilm is cultivated adjacent the aerobic biofilm and thewastewater to be treated is maintained in an anaerobic state. A sourceof agitation 98 is operated from time to time to agitate the sheets 26to release accumulated biofilm. A suitable source of agitation is aseries of mechanical mixers.

[0100] Referring to FIG. 17, a reactor 100 has a tank 112 with one ormore membrane supported biofilm module cassettes 110 installed inside ofit. The cassettes may have one or more modules 40, as described above.The module 40 may also be a tow module, a module of planar elements, orother types of modules using a membrane to support a biofilm. Eachmodule 40 has a gas inlet header 116 fed with air, or another oxygencontaining gas, through a blower 118. Gas passes from the inlet header116 to the inside (or lumens 14) of one or more fibers 10. The walls ofthe fibers 10 serve as gas transfer membranes 120. A portion of the gaspasses through the membranes 120 while another portion, and possiblysome gasses taken up from the tank 112, flow to an outlet header 122 ofthe modules 40 and to an exhaust 124. The gases leaving the exhaust 124may be post-treated or discharged to the atmosphere.

[0101] Feed water enters the reactor 100 through a feed valve 126 andfeed pump 128. The feed is filled to a feed fill level 130 above themodules 40. After a batch of feed has been treated, a drain valve 131 isopened to drain the tank 112 of treated water. The treated water mayflow to a municipal sewer, to the environment, discharge directly to areceiving stream, or to another stage of a MSBBR (membrane supportedbiofilm batch reactor) or to another sort of reactor for furtherprocessing.

[0102] A biofilm 132 grows on the outside of the membranes 120. Tocontrol the thickness of the biofilm 132, one or more aerators 134 areprovided below the modules 140 and connected to a scouring air blower136 through an aeration valve 138. The scouring air blower 136 can beoperated to provide bubbles when the tank 112 is full of water. Thebubbles rise through the module 140 and physically remove some of thebiofilm 132 from the membranes 120. The aerators 134 are also attachedto a gas supply 140 through a gas supply valve 142. The gas supply 140may contain a pressurized gas or a gas generator and pump or otherdevice for supplying a gas when the tank 112 is empty. The reactor 100also has a liquid pump 144 operable to fill the tank 112 with a liquidother than feed water. The liquid pump 144 may be connected to areservoir holding the liquid or to a source of clean water passingthrough a modifier, such as a chemical injection device or heater. Thetank 112 is generally open to the atmosphere and contains liquid atgenerally ambient pressure but has a lid 146 which may be closed fromtime to time to provide an enclosed space.

[0103] The main treatment process in the reactor 100 involves the batchapplication of feed to the biofilm 132. The tank 112 is filled with feedto the feed level 130 using the feed pump 128. The feed pump 128 isconnected to the feed supply through an equalization reservoir 148 topermit batch operation from a non-batch feed. The feed remains in thetank 112 for a period of time, for example between 12 and 96 hours,while it is treated by the biofilm 32. During treatment, the lid 46 mayremain open, but the water in the tank 112 is generally anoxic oranaerobic. However, oxygen, typically as a component of air, is suppliedto the biofilm 132 through the membrane 120 by the blower 118 creatingan aerobic region on the biofilm 132. From time to time during thetreatment period, a recirculaton valve 149 may be opened and feed pump128 operated to mix the feed water in the tank 112.

[0104] After the biofilm 132 has digested the feed to the desireddegree, the drain valve 131 is opened to drain the tank 112. Thedraining may occur in two steps. In the first step, the solids slurrypresent in the bottom of the tank is drained to remove settled solidswhich are then transferred to a sludge management system. In the secondstep, the clear decanted liquid is then drained to a second stagetreatment or disinfection system, or discharged to a sewer, ordischarged to a receiving stream.

[0105] The oxygen bearing gas supply may be continued throughout thefilling operations to continue digestion of the material adsorbed on thebiofilm, and to ensure that treatment starts immediately as soon as aportion of the biofilm is immersed in the wastewater. Similarly aerationmay continue throughout the draining operation to continue treatment aslong as a portion of the biofilm is immersed and to digest organicsadsorbed in the biofilm for a short period of time even while notimmersed, so as to maximize the time of treatment of each batch.

[0106] Referring now to FIG. 18, a reactor 400 is shown having similarfeatures as the reactor 100, but without the gas supply 140, gas supplyvalve 142, or liquid pump 144.

[0107] In a batch process, the concentration of the wastewater decreasestowards the end of each processing period. Demand for oxygen supplied tothe biofilm also decreases and so the gas supply to the modules may bereduced. Modules using fibres at least partially in the form of towsallow a very high surface area for oxygen transfer and biofilm growth.Tow modules are particularly useful in treating wastewater having a lowCOD, for example 1,000 mg/L or less, 500 mg/L or less or 300 mg/L orless, because they provide large surface areas. Pressure loss throughthe fine fibre lumens is not limiting with the amount of air supplyrequired to deliver oxygen to a biofilm treating low COD wastewater.Although they may be useful for treating other wastewaters as well, towmodules can be used where the initial feed has a low COD or as a secondor third stage behind other treatment processes or apparatus that reducethe COD concentration of stronger feedwaters. With municipal wastewateror other feeds, for example feeds having a COD of 1,000 mg/L or more, atwo stage apparatus may be used. In a first stage, membrane supportedbiofilm modules in the form of a fabric sheet are used as in FIG. 9. Theoutlet from a reactor containing these modules is fed to a reactorcontaining tow modules with sheets as in FIG. 4 which provides secondstage treatment. The inventors have observed that rapid reduction in CODfrom a high COD wastewater limits the denitrification produced from amembrane supported biofilm reactor. With a two stage process, the firststage may be optimized for COD removal. The feed to the second stage hasa reduced COD and the second stage may be optimized to supportnitrifying microorganisms, for example of the species nitrobacter andnitrosomas, over carbon degrading microorganisms to provide improvedammonia oxidation in the second stage.

[0108] In general, when considering COD, soluble COD is used sincesoluble COD is most easily digested by a biofilm 30 and is easilymeasured. However, particularly for modules 40 with loose tows 20 oversome or all of their area, some particles of insoluble COD are trappedin the biofilm. Over time, these particles are broken down into solubleCOD and digested. Accordingly, total, or total biodegradable, COD alsomay be a relevant parameter in some embodiments.

[0109] For feeds having a CODs of 1000 mg/L or more, a module 40 mayhave an SA_(OXYGEN)/SA_(BIOFILM) of 1 or more, for example between 1 and10. Modules 40 having sheets 26 woven across the entire length of thefibers 10, in a dense weave with a high number of fibers for very highloadings, for example, are useful. For feeds having a CODs of 1000 mg/Lor less, a module 40 may have an SA_(OXYGEN)/SA_(BIOFILM) of between 0.2and 2.5. Modules 40 having sheets woven across the entire length of thefibers but with a less dense weave, or sheets 26 with a central open tow20 area, for example, are useful. For feeds having a CODs of 300 mg/L orless, a module 40 may have an SA_(OXYGEN)/SA_(BIOFILM) of 1 or less, forexample between 1 and 10. Modules 40 with sheets 26 have a central opentow 20 area, or modules 40 of loose tows 20, for example, are useful.

[0110]FIG. 19a shows a bench scale reactor having a module 40 made bypotting 100 tows 20, each of 96 fibres 10 as shown in FIG. 1, into anopposed pair of headers 44. The module 40 was used to treat a feed waterin a batch process. In the process, the module 40 was located in a tank112 filled to 4 L of synthetic wastewater. The tank was drained andfilled with fresh feed every 1 to 7 days. Air was applied to the moduleat 10 mL/min. A biofilm 30 of stable thickness grew on the module 40 fora period of over 6 months. The biofilm 30 was essentially endogenous,its rate of growth generally equal to its rate of decay, except that asmall part of the biofilm 30 broke off and was discharged with some ofthe tank drains. A section of a tow 20 is shown in FIG. 19b. Individualfibers 10 are covered in biofilm 30. In some places, the biofilm 30around a small group of fibers 10 may merge together for a portion ofthe length of the fibers 10. The thickness of the biofilm 30 shown isabout 250 microns.

[0111] Referring now to FIG. 20, another reactor is shown as suitable,for example, for a septic tank, septic tank retrofit or shipboardtreatment plant. The particular reactor shown is a septic tank retrofitusing a standard septic tank 410 with an inlet 412 and an outlet 414 onopposite sides. The tank 410 has two stages including a primary chamber416 and a secondary chamber 418. A dividing wall 420 has a submergedorifice 422 that allows flow between the chambers 416, 418. One or moremodules 424 are placed in the secondary chamber 418. Air is supplied tothe bottom headers of the modules 424 through inlet tubes 426. Exhaustair is vented from the upper headers of the modules 424 through exhausttubes 428. Scouring air is periodically applied to a sparger 430 locatedunder or near the bottom of the modules 424 through scouring air tube432. The modules 424 each have 1 to 100 or 8-20 sheets as in FIG. 4potted into a pair of headers to produce a module 424. For example, aseptic tank for a single household may have one 8 to 10 sheet module 424fed with a ¼ hp air blower and creating a pressure drop of about 1 to 7psi, or about 3 psi. With a typical household feed, a generallyendogenous biofilm grows on the individual fibre 19 and tow 20 surfaces.Biological treatment in the biofilm results in a reduction in thesuspended solids and chemical oxygen demand of the effluent, allowingthe septic tile field to be reduced in size or eliminated.

[0112] In another embodiment of the invention, a number of bioreactorsare installed in series to provide flow patterns approaching plug flow.This results in higher reaction rates and better utilization of oxygen.

[0113] In another embodiment of the invention, different oxygen levelsare used in different stages of the bioreactor by oxygen spiking to meetdifferent levels of oxygen demand and to achieve high bioreactorloadings. Different oxygen levels may also be used at different times ina single reactor or stage of a reactor. To increase the oxygen level,the pressure of the gas fed to the lumens of the fibers or the oxygencontent of the feed gas can be increased. Similarly, to decrease theoxygen level, the feed gas pressure or oxygen content can be decreased.Higher oxygen levels may be used in upstream stages of multi-stagereactors or in highly loaded reactors. Oxygen levels may also beincreased periodically or from time to time to correspond to periods oftime when the loading on a reactor is temporarily increased, for exampleto respond to seasonal or daily variations in wastewater strength orquantity.

[0114] 3.0 Biofilm Control

[0115] In a membrane supported biofilm reactor, it can be advantageousto control the thickness of the biofilm on the membranes. For example,in the reactor 100 (FIG. 17), although the tank 112 is drainedperiodically, most of the biofilm 132 remains on the membranes 120,particularly where the feed has a high COD, for example over 300 mg/L.Excess thickness of the biofilm 132, for example 2 mm thick or more,provides minimal, if any, increase in digestion rate, over a thinnerlayer, for example of 1 mm thick or less. However, keeping the biofilm132 thin allows the sheets 26 of the modules 40 to be placed closertogether, providing more surface area per module volume. This increasein surface area generally more than offsets any minor increase indigestion that may, or may not, be achieved with a thicker biofilm 132.

[0116] Accordingly, means are provided to prevent the biofilm 32 frombecoming unnecessarily thick. The following methods may be performedindividually or in various combinations. The frequency of treatmentvaries with the growth rate of the biofilm 132. For example, a biofilm132 may grow by 10 microns a day and the module 40 may be made totolerate a biofilm of between 0.2 mm and 0.8 mm. Biofilm controlprocedures may then be required every 5 to 10 days. Alternately, theperiod between biofilm control procedures may be linked to the amount ofCOD that the biofilm has digested since the last control procedure,which is in turn related to the time and biofilm thickness increasesince the last control procedure. For example, control procedures may beperformed when the biofilm has digested about 20 to 200 grams of CODsper square meter of biofilm area since the last control procedure. Whencontrol or thickness reducing procedures are performed at thisfrequently, a stable biofilm layer is maintained over extended periodsof time even though each control period does not have a drastic effecton biofilm thickness. Control procedures may be applied to the entirebiofilm at once or to a portion of the biofilm at a time.

[0117] 3.1 Mechanical Methods of Biofilm Control

[0118] Some methods for controlling the thickness of the biofilm 132 onthe membranes 120 involve mechanically removing part of the biofilm 132.In one such method, still referring to FIG. 17, one or more aerators 134are provided below the modules 114 and connected to a blower 136 throughan aeration valve 138. With the tank 112 full of liquid, blower 136 isoperated to create bubbles from aerator 134 below the modules 40. Thebubbles mechanically scour the biofilm 132 and also create a flow ofwater through the modules 40 that physically removes some of the biofilm132. A high velocity of scouring air of 2-8 feet/second or an airapplication rate of 5 to 20, for example about 10, cubic meters per hourper square meter of module footprint for intervals of 1 to 10 minutesmay be used. This may be done, for example once every day to once everyweek. Also, air may be used to periodically mix the contents of thebioreactor.

[0119] Other mechanical methods include spraying the modules 40 withwater while the tank 112 is empty and physically removing biofilm 132such as with a comb, wire or brush. The removed biofilm 132 falls to thefloor of the tank 112 and may be flushed out through drain 131 forfurther processing as for waste sludge. These mechanical methods may beperformed less frequently than other methods and, when performed, may beperformed after another method has weakened the biofilm 132.

[0120] Mechanical methods for controlling the biofilm are enhanced byproviding the sheet 26 with a rough or textured surface, the height ofthe surface undulations being in the range of the desired biofilmthickness. Desired biofilm thickness may be 200 to 1,000 microns.

[0121] 3.2 Chemical Methods

[0122] In another embodiment, ozone gas, introduced in the fibre lumenis used to oxidize a part of the biofilm to make it digestible. Oxygenis then provided to the lumens to permit the biofilm to digest theoxidized organics, thereby reducing the total amounts of solidsgenerated and to control the biofilm thickness. The oxygen may beprovided as a separate step or as part of the regular step of digestingwastewater. The reactor may be treated in this way one module or sectionat a time.

[0123] In another method, a control substance is applied to the tankside of the biofilm 132. For example, after the tank 112 is drained,clean water heated to, for example, 35-55 C, may be pumped into the tank112 by the liquid pump 144. The heated water is kept in the tank 112 fora period of time (contact period), for example 3-5 hours, sufficient tokill a fraction of the biofilm 132 and dissolve some of the organicsthat form the biofilm matrix. The biofilm is also starved to some extentsince feed has been removed. Oxygen may continue to be applied to thelumens or may be turned off. Air scouring may also be provided duringthis period to enhance biofilm removal, although it may be moreeconomical to carry out this operation without air scouring,particularly if the blower 136 and aerator 134 can then be eliminatedfrom the reactor 100 entirely. The biofilm 132 is also starved to someextent. After the contact period, the water is drained through drainvalve 131. In an industrial treatment system, the discharge water willhave some COD but the duration of the contact period can be chosen suchthat the discharge is still suitable for discharge to a municipal sewersince most of the killed organisms will remain in the biofilm 32. Duringa later part of the contact period, the living inner part of the biofilm32 will biodegrade the killed organisms. The effect of the heated water,or unheated water, may be enhanced with the addition of chemicals suchas acids, for example with a pH between 1 and 6 or between 3 and 3,bases, for example with a pH between 8 and 13 or between 9 and 11, orenzymes. The chemicals and their concentration and contact time arechosen to partially dissolve or weaken some organics that are structuralcomponent of the biofilm but to kill only a fraction of themicroorganisms while leaving the majority behind in an active biofilmfor rapid restart of the reactor.

[0124] In another method, a gaseous control substance is applied to thetank side of the biofilm 132. The gas is applied from gas supply 140while the tank 112 is drained at the end of a batch cycle. Lid 146 isclosed so that the gas remains in the tank 112. The gas may be ofvarious types, for example an acid such as chlorine. Alternately, ozonemay be used. The primary purpose of the ozone is to break up the cellwalls of the microorganisms in the biofilm 132 to make it morebiodegradable. The amount of ozone applied would not be sufficient tooxidize more than about 5% of the biofilm directly and to kill only afraction of the microorganisms present in the biofilm. However,refractory organic material is converted to organic material which islater reduced by biological oxidation when the tank is refilled. Theozone is generated in a gas phase (air or oxygen) and is easilydispersed in an empty tank 112. The ozone is kept in the tank 112 for aperiod of time allowing it to be absorbed by the biofilm 132. Redoxconditions can be controlled in the tank 112 while it is drained topromote sludge reduction. Alternating aerobic and anaerobic conditionscan be established in the biofilm 132 by turning the feed to the inletheader 116 on and off while the tank 112 is filled with ozone to enhancethe effects of the ozone. Killed and partially oxidized organisms remainin the biofilm 132 and are later digested in situ such that excessbiomass need not be removed from the tank 112 for further treatment.Denitrification may also be improved because the carbon/nitrogen (C/N)ratio increases. Ozone may also be used in this method with membranes120 that are sensitive to ozone since the membranes 120 are protected bythe biofilm 32.

[0125] 3.3 Biological Methods

[0126] In another method, worms or other animals or higher life formsare used in an isolated section of the reactor to digest excess biofilmto reduce bio-solids generation. The worms etc. are grown in a separatebioreactor. When desired, the worms etc. are applied to the biofilm byfilling the tank with a liquid suspension or brine containing the wormsetc.

[0127] Another method of biofilm control is endogenous respiration. Bythis method, the feed loading applied to the biofilm 132 is kept suchthat the rates of decay of the biofilm 132 equals its rate of growth. Inpractice, the rate of growth may exceed the rate of decay by a smallamount in a batch process because some of the biofilm 132 may detach andleave the tank 12 when it is drained. However, endogenous respirationoccurs practically only at low loading rates and so is more appropriatefor feeds with low COD concentrations, for example 1000 mg/L CODs orless or 300 mg/L CODs or less.

[0128] Another method is periodic starvation. In this method, the feedis kept in the tank 112 for an extended period of time such that the CODconcentration drops to below what it is at the end of a typical batchprocess. The biofilm 132 is not nourished and decays rapidly until thestart of the next batch cycle. The biofilm can also be starve byremoving the feed and filling the tank with clean, for example tap orpotable, water, or by loading the reactor at less than 0.1 kg CODs perkg MLSS per day.

[0129] In another method, the supply of gas to the inlet header 116 ofthe module 40 is turned on and off cyclically for a period of time. Thevarying supply of oxygen shocks the biofilm 132 and causes increaseddecay. Aerobic and anaerobic areas in the biofilm expand and contractwhile consuming, or being consumed by, the other. Alternately, gasessuch as ozone or chlorine, may be added to the inlet header 116 toenhance the shock.

[0130] With chemical or biological biofilm control, closer spacingbetween the sheets 26, for example 3-4 mm, may be used since hydraulicflow through the modules 40 is not required as with air scouring,agitation or other physical methods of biofilm removal. Chemical orbiological methods are also useful where sheets 26 or fibers 10 or units19 are not arranged so that a flow of scouring air will not reach allparts of the biolfim. Chemical or biological biofilm control methods mayalso be useful with open sheets 26 or modules with unsupported or loosefibers 10, fiber units 19 or tows 20 that would be damaged by airscouring, agitation or physical methods. Alternately, one or morechemical methods, one or more mechanical methods or one or morebiological methods may be combined.

EXAMPLES Example 1 Chemical Oxygen Demand (COD) Reduction in a MembraneSupported Bioreactor

[0131] A bench scale bioreactor was made using a module generally aspresented in FIGS. 6-9 except that only a single sheet of the fibres wasused. The length of the sheet was 0.57 m and height 0.45 m, providing atotal biofilm area of approximately 0.5 m² assuming a with both sides ofsheet available for biofilm growth. The ratio of surface area for gastransfer to surface area of attached biofilm was between about 5 and 6.Inlet air flow was 25 ml/min at a pressure of 34.5 kPa. Reactor volumewas 30 L. Synthetic wastewater with a COD level of 1000 mg/l wasintroduced in a batch manner periodically. The synthetic wastewaterconsisted of 1.0 g/L of soluble peptone and 0.03 g/L of sodium hydrogenphosphate dissolved in tap water. A series of batch reactions wereconducted to determine the rate of reaction and oxygen transferefficiency. FIG. 21 presents the results of three batch periods: a threeday period form day 2 to day 5, a three day period from day 6 to day 9and a one day period from day 9 to day 10. It can be seen that 80-90%reduction of COD was obtained in each of the three-day batch periods. ACOD reduction of about 40% was achieved in the one-day batch periodsuggesting that the rate of COD reduction is higher while theconcentration of wastewater is higher and that the COD reduction ratelevels off as the COD concentration in the batch decreases with time.Oxygen transfer efficiency during these series of tests ranged from 50to 70%, as measured by the exit concentration of air.

Example 2 Bench Test with Synthetic Wastewater

[0132] A bench scale bioreactor was designed using a single sheet moduleas described for Example 1. Synthetic wastewater with a COD level of1000 mg/l, as described in Example 1, was introduced and treated by thebiofilm on the module. Rates of COD removal and oxygen transfer and thethickness of the biofilm were calculated or measured and recorded. Forabout the first 21 days, the reactor (which has a 30 L fill volume)drained and re-filled with feed after variable batch periods to keep theCODs in the tank generally between 500 and 1000 mg/L. At day 8 and day16, in addition to emptying the tank and re-filling it with new feed,the module was powerwashed with a water sprayer to remove biofilm. Fromabout day 21 to day 30, the biofilm was subjected to starvation (i.e.the tank was filled with tap, i.e. clean or drinkable, water whileoxygen supply continued to the module) and air scouring treatments. Onabout day 30, the tank was emptied and re-filled with feed. From thenon, the tank was emptied and re-filled with wastewater daily but nobiofilm control steps were taken, to allow the biofilm to grow inthickness and observe the effect and rate of such growth. The results ofthe test are presented in FIG. 21. It can be observed that the CODremoval rate varied between about 19 to 38 grams per square metre perday without being proportional to the biofilm thickness. The oxygentransfer varied between about 10 to 15% grams per square metre per day,also over a relatively wide range of biofilm thickness, namely, fromabout 0.5 mm to over 2.3 mm, at which thickness the measurement devicereached its maximum thickness.

Example 3 Pilot Study with Industrial Wastewater

[0133] A small pilot study was conducted using four modules generally asshown in FIGS. 6 to 9. Each module has 6 sheets of fibers and a totalplanar surface area, or area of biofilm, of about 3.6 m², and a ratio ofsurface area for gas transfer to surface area of attached biofilm ofbetween about 5 and 6. The modules were installed in a 300 litre tank.The reactor was initially operated with peptone (about 2000 mg/l) andthen peptone added to wastewater in a declining ration to accelerate theinitial growth of biofilm on the sheets but then acclimatize the biofilmto the wastewater. After acclimatizing the biofilm, batch operationswere conducted, filling the tank with industrial wastewater. Thewastewater was drawn from multiple sources in ratios chosen to create anfeed COD of about 3000 mg/l. “Pure” oxygen was supplied to the modulesat a feed pressure of about 5 psi. As shown in FIG. 23, bulk CODsconcentration dropped to less than 1000 mg/l in about 2 to 3 days. Itwas also noted that COD removal rates declined with bulk CODsconcentration in the wastewater, and with time, during each batch.

[0134] COD removal rates were calculated at different periods of timeduring the batches corresponding to different concentrations of CODs inthe tank. Batches having initial CODs of 5000 mg/l and 7000 mg/l werealso tested to observe the effect of higher initial COD concentrationson COD removal rate. The results are presented in FIG. 24. As indicatedin FIG. 24, removal rate was generally higher at higher loadings exceptthat, in the reactor tested, very high loadings did not always producevery high removal rates suggesting that one or more of air feedpressure, surface area for air transfer to biofilm surface area or totalmodule area were less than optimum for very high loadings.

[0135] The same reactor was used for a series of trials conducted undercontinuous operation. In the trials, HRT and inlet CODs were varied. Thefeed gas was “pure” oxygen at a feed pressure of 5 psi. For each trial,the average inlet CODs, outlet CODs and removal rate, organized by HRTof the trial, are presented in FIG. 25. COD removal rates generallydecreased as HRT increased or as inlet CODs decreased.

[0136] The effectiveness of biofilm control procedures were alsoverified in the reactor during the batch trials mentioned above. Gentleaeration of about 1 scfm/module for 15 seconds every hour was applied,primarily for mixing, and more aggressive air scouring of about 4scfm/module for 2-3 minutes every 2-3 days was applied primarily toremove biofilm. The biofilm thickness was successfully maintained in arange from about 0.2 mm to less than 0.8 mm regardless of the averagebulk CODs in the reactor, which varied from about 300 mg/L to about5,500 mg/L.

Example 4 Pilot Study with Municipal Wastewater

[0137] Another pilot study was conducted using two modules as describedin Example 3, each having a surface area of about 3.6 m², installed inan 85 litre tank. Air was supplied to the modules at a feed pressure ofabout 5 psi. Peptone was added initially to the sewage to accelerate theinitial growth of biofilm on the sheets as described for example 3.Batch operations were conducted, filling the tank with municipalwastewater, screened through a 3 mm screen, having an initial CODs ofaveraging about 100 to 200 mg/l, but occasionally up to 700 mg/L. At theends of the batches, CODs concentration had generally dropped to lessthan 30 mg/l and COD removal rate had also generally dropped to lessthan 1 g/m2/d. The levels of CODs and COD_(t) with respect to timewithin a sample period in a batch are presented in FIG. 26.

[0138] A study was also conducted with a continuous process, withdifferent trials performed over a total period of about 60 days. In thetrials, HRT varied from 24 hours to 3 hours and inlet CODs from 100 mg/lto 200 mg/l. Average removal rates tended to be lower with lower loadingrates.

[0139] Nitrification and denitrification kinetics were also measured inthe continuous process study. The results of 4 trials are presented inthe following table. TABLE 1 Nitrification and Denitrification inContinuous Operation Inlet Inlet Outlet Outlet Outlet HRT CODs NH3-NCODs NH3-N NO3-N (hr) (mg/L) (mg/L) (mg/L) (mg/L) (mg/L) 11.5 165 18.229 3.5 3.4 7.8 117 19.6 25 5.4 4.4 4.4 105 17.7 35.9 5.6 4.3 3.1 84 18.737.6 11.6 1.3

[0140] Biofilm control was also tested in the municipal wastewaterstudy. Biofilm thickness averaging 0.2 mm was observed with airscouring, but thicker biofilm appeared to collect between someindividual sheets indicating that these areas were not receiving fullscouring air.

Example 5 Bench Scale Study with a Tow Module with Wastewater

[0141] A module similar to the one shown in FIG. 5, having 100 PMP fibretows, each tow having 96 fibres of dense walled PMP, was tested. Thetotal surface area of the fibres in the module was 0.54 m². In themodule, each tow was individually potted into an upper and lower header.The module was fed with a supply of air at a rate of 10 ml/min to thebottom header and exhausted out of the top header. The module wassuspended, with the top header held in a clamp at the water surface andthe bottom header weighed down, in a container filled to a volume of 4L. The module was operated in a batch mode using a synthetic wastewaterof 1000 mg/L CODs and also wastewater from a septic tank. At the startof each batch processing period, the container was filled withwastewater. Air was supplied to the module to support a biofilm growingon the fibres for processing periods ranging from between about 1 to 7days while wastewater was neither added to nor withdrawn from the tank.Shorter batch periods were generally used with wastewater having lowerconcentrations of COD. At the end of the processing period, the tank wasdrained. New wastewater was added to start the next processing period.At various times, the module was removed to non-destructively measurethe thickness of the biofilm on them and measurements of the COD in thewastewater were taken.

[0142] The thickness measurements from the tests using syntheticwastewater are recorded in FIG. 27 which shows the thickness of thebiofilm on the fibres over the period of 180 days of operation. Therewas initially no biofilm but after about 20 or 40 days a biofilm haddeveloped having a thickness that generally ranged between about 100 and300 μm. For most of the test run, no additional methods were used tocontrol the biofilm thickness and yet the biofilm thickness remainedgenerally stable and acceptable. Small portions of biofilm were observedto be shed from the module during at least some of the tank drainingoperations, and biofilm control was otherwise provided by endogenousgrowth of the biofilm. However, for a period of approximately 15 days,the module was operated in a starvation mode. In this mode, the tank wasfilled with tap water and air feed was continued. The biofilm wasreduced in thickness from about 250 μm to about 100 μm during thestarvation period indicating that the starvation period was effective atreducing the thickness of the biofilm.

[0143]FIGS. 28 and 29 show the removal rate of COD in tests using thesynthetic wastewater. FIG. 28 shows removal rate as a function of timeand FIG. 29 shows removal rate as a function of COD concentration.Referring first to FIG. 28, each vertical line within the figureindicates the start of a new batch processing period. Accordingly, atthe times indicated by the vertical lines, new wastewater having a CODof 1,000 mg/L was added to the tank. As the batch progresses, thewastewater is treated and accordingly its COD concentration reduces. Asshown in FIG. 28, the COD removal rate tended to drop with time in eachbatch processing period suggesting that the removal rate is related tothe COD concentration in the wastewater. Further, the removal rate inthe batch between day 154 and day 159 approached zero indicating thatfurther processing time would have marginal value. In FIG. 29, the CODremoval rate is plotted directly against the average COD concentrationin the wastewater. As indicated in FIG. 29, the relationship between CODremoval rate and COD concentration in the wastewater is nearly linearwith the removal rate being generally proportional to the CODconcentration.

[0144] For the tests using septic tank wastewater, the wastewater wastaken from the second chamber of a septic tank. For one trial, thecharacteristics of the wastewater were as follows:

[0145] Total Chemical Oxygen Demand (COD_(t)): 377 mg/L

[0146] Soluble COD (COD_(s)): 199 mg/L

[0147] Ammonia Nitrogen (AN): 55.1 mg/L

[0148] Total Suspended Solids (TSS): 70 mg/L

[0149] The module was operated in a batch mode with batch processingperiods of approximately 24 hours to simulate actual reaction conditionsin a septic tank. Air was supplied during these periods at the rategiven above to provide oxygen to the biofilm. After one processingperiod of 22 hours and 35 minutes in duration, a sample of the treatedwastewater was analyzed and results were as follows:

[0150] COD_(t): 140 mg/L

[0151] COD_(s): 73 mg/L

[0152] AN: 24.7 mg/L

[0153] TSS: 1 mg/L

[0154] A significant improvement in effluent quality was achieved. Inparticular, a huge reduction in TSS was achieved. By visual observation,a large portion of the TSS removed was in the form of colloidal matter.

[0155]FIG. 30 records the results from another trial using septic tankwastewater. The reactor was operated for a two-day batch period withconcentration of COD_(t), CODs TSS and ammonia nitrogen measured at thebeginning, middle and end of the batch period. For comparison purposes,another sample of wastewater taken from the same septic tank on the sameday was placed in a 500 mL graduated cylinder and monitored as acontrol. After two days of operation, reduction of Total COD (COD_(t))in the reactor approached 75 mg/L, with a removal in excess of 70%. TSSdropped from 34 mg/L to almost no appreciable TSS after two days oftreatment. Ammonia was also reduced during this period. During the sameperiod, the control had a less than 40% reduction in COD and an increasein TSS. The batch process and reactor effectively treated the septictank wastewater by removing COD but also removing suspended solids, inpart because of the quiescent nature of the process.

Example 6 Chemical Biofilm Control

[0156] A biofilm control study was done using the single sheet reactordescribed in Example 1 with a very thick biofilm on it. At the start ofthe test, the tank was drained and 30 L of sodium hydroxide solution indeionized water at a pH of 9.43 and a temperature of 40 C was added tothe reactor. After a first 4 hours of soak, air scouring at 2 scfm wasstarted and was continued for more than 18 hours while sodium hydroxidesolution remained in the tank. Air supply to the lumens remained on. Thebiofilm thickness was reduced slightly (4.6 mm to 4.3 mm) over the firstfour hour period. After the 18 hours of soaking and air scouring, thethickness of the biofim was reduced further to 3.2 mm.

[0157] In another biofilm control study, 6 single sheet modules as shownin FIG. 10a and 10 b were used. Each sheet was about 27 cm long by 20 cmwide and had an available surface area of about 0.11 square meters. Thesheets were woven with the hollow fibers running lengthwise and open atboth ends. The ratio of air transfer area to biofilm area was about 6to 1. The modules were placed in a 20 L (working volume) reactoroperated in batch mode at room temperature with batch periods of about 3days. The reactor was fed with synthetic sewage at concentrations from2000 to 8000 mg/L CODs. Air was fed to the lumens of the modules atabout 2 psi with a flow rate of about 20 mL/min to an inlet header ofeach sheet. At intervals of from 3 to 7 days, between batches, themodules were soaked for 4 hours in a solution of NaOH in hot water witha pH of 10 at 50 C. Air supply to the lumens remained on. After the 4hours, the reactor was re-filled with feed. No air scouring was providedduring the soak periods or during the batch periods. FIG. 31 shows thebiofilm thickness over time which was maintained between 0.2 and 0.8 mmand averaged about 550 microns over a 140 day period. Calculated resultsfrom the batches during that period indicate that during the intervalbetween cleanings the biofilm removed from 66 to 120 grams of CODs persquare meter.

[0158] Many modifications and variations of the present invention arepossible within the teachings of the invention and the invention may bepracticed other than as described above. The scope of the invention isdefined by the following claims.

We claim:
 1. An apparatus for supporting a biofilm in a liquidcomprising: a) a plurality of gas permeable hollow fibers, each hollowfiber having a lumen, an outer surface and an open end; and, b) aheader, the header having a cavity and a port open to the cavity,wherein the hollow fibers extend from the header, with the outersurfaces of the open ends of the hollow fibers sealed to the header andthe lumens of the hollow fibers communicating with the port through thecavity.
 2. The apparatus of claim 1 wherein the hollow fibers have anoutside diameter of 100 microns or less.
 3. The apparatus of claim 1wherein the hollow fibers have a hollow area of 10% or more, morepreferably 30% or more.
 4. The apparatus of claim 1 wherein the hollowarea is 50% or less.
 5. The apparatus of claim 1 wherein the hollowfibers are non-porous or dense walled.
 6. The apparatus of claim 1wherein the hollow fibers comprise polymethyl pentene.
 7. The apparatusof claim 1 wherein the hollow fibers have a second end and are between0.25 metres and 3.0 metres long.
 8. The apparatus of claim 7 wherein thehollow fibers have a second end and are between 1.0 metres and 2.0metres long.
 9. The apparatus of claim 1 wherein the hollow fibers arearranged into groups.
 10. The apparatus of claim 9 wherein the groupscomprise between 24 and 96 hollow fibers.
 11. The apparatus of claim 9wherein the groups further comprise second fibers that are stronger thanthe hollow fibers.
 12. The apparatus of claim 9 wherein the group is atow of fibers.
 13. The apparatus of claim 9 wherein the group is athread, yarn or twisted fibers.
 14. The apparatus of claim 1 wherein thehollow fibers are curled, crimped or undulating along their length. 15.The apparatus of claim 1 wherein the hollow fibers extend along theirlength generally in a first direction.
 16. The apparatus of claim 15further comprising third fibers extending along their length generallyin a second direction, the second perpendicular to the first direction.17. The apparatus of claim 16 wherein the third fibers and hollow fibersare intertwined.
 18. The apparatus of claim 17 wherein the hollow fibersand third fibers form a fabric.
 19. The apparatus of claim 18 whereinthe fabric is generally continuous across the length of the hollowfibers.
 20. The apparatus of claim 18 wherein the fabric extends over aportion of the length of the hollow fibers near their open ends and doesnot extend over a central portion of the length of the fibers.
 21. Theapparatus of claim 20 wherein the hollow fibers and third fibers arewoven, knitted, stitched or warp knitted together over at least aportion of the length of the hollow fibers.
 22. The apparatus of claim 1wherein the hollow fibers have second open ends.
 23. The apparatus ofclaim 22 wherein the second open ends of the hollow fibers are potted ina second header.
 24. The apparatus of claim 23 wherein the second openends communicate with a second port of the second header through asecond cavity of the second header.
 25. The apparatus of claim 23wherein the header and the second header are spaced apart from eachother and the hollow fibers are arranged into one or more flat sheets orgenerally parallel planar structures extending between the headers. 26.The apparatus of claim 25 wherein the flat sheets or planar structuresare generally parallel to each other.
 27. One or more of the apparatusof claim 25 wherein adjacent planar structures have a spacing betweenthem of between 2 mm and 20 mm or, more preferably, of between 3 mm and15 mm.
 28. The apparatus of claim 23 wherein the first header and secondheader are held apart at a distance that applies a tensile force to thehollow fibers.
 29. The apparatus of claim 25 further comprising spacersbetween the flat sheets or planar elements outside of the header. 30.The apparatus of claim 25 wherein the flat sheets or planar structuresfurther comprise a rigid member extending between the headers.
 31. Theapparatus of claim 1 having a surface area for oxygen transfer tosurface area of supported biofilm ratio of about 1.6 or more.
 32. Theapparatus of claim 31 having a surface area for oxygen transfer tosurface area of supported biofilm ratio of about 2 or more.
 33. Theapparatus of claim 32 having a surface area for oxygen transfer tosurface area of supported biofilm ratio of about 5 or more.
 34. Theapparatus of claim 33 having a surface area for oxygen transfer tosurface area of supported biofilm ratio of about 1 or less.
 35. Theapparatus of claim 18 wherein the roughness of the fabric is between 0.1and 2 mm.
 36. A reactor comprising: a) a tank for holding a liquid to betreated, the tank having an inlet and an outlet; b) an apparatusaccording to claim 1; and, c) a gas delivery system for providing a gasto the port.
 37. The reactor of claim 36 further comprising an agitatoror aerator adapted to agitate the liquid around the apparatus.
 38. Thereactor of claim 36 further comprising a chemical injection system forinjecting chemicals into either the lumens of the hollow fibers or apart of the reactor in communication with the outer surfaces of thehollow fibers.
 39. The reactor of claim 36 having a heater to heateither the gas provided to the port or the liquid held in the tank. 40.A multi-stage reactor having two or more reactors according to claim 36,the outlet of a first reactor connected to the inlet of a secondreactor.
 41. The multi-stage reactor of claim 40 wherein the first andsecond reactors are plug flow reactors, batch reactors or continuouslystirred reactors.
 42. The multi-stage reactor of claim 37 wherein theapparatus of the second reactor has a lower surface area for oxygentransfer to surface area of supported biofilm ratio than the apparatusof the first reactor.
 43. The multi-stage reactor of claim 40 whereinthe apparatus of the first reactor has a surface area for oxygentransfer to surface area of supported biofilm ratio between of 5 or moreand the apparatus of the second reactor has a surface area for oxygentransfer to surface area of supported biofilm ratio of 5 or less. 44.The reactor or multi-stage reactor of claim 36 wherein the reactor(s)have a plurality of the apparatus arranged in parallel between the inletand outlet.
 45. The multi-stage reactor of claim 40 wherein the fibersof the apparatus of the first reactor are formed into a sheet alongtheir entire length while the fibers of the apparatus of the secondreactor are unsupported by perpendicular fibers over a portion of theirlength.
 46. A process for treating a liquid comprising the steps of: a)contacting an apparatus having a port in communication with one or moreinner surfaces of a gas permeable biofilm support medium with theliquid; and, b) providing a gas to the port of the apparatus, the gaspermeating to outer surface(s) of the medium to support a biofilmgrowing on the outer surface(s).
 47. The process of claim 46 wherein theliquid comprises wastewater.
 48. The process of claim 46 wherein the gascomprises oxygen.
 49. The process of claim 46 wherein the gas compriseshydrogen.
 50. The process of claim 47 wherein the biofilm is maintainedin an aerobic state adjacent the outer surface(s) and in an anoxic oranaerobic state adjacent the liquid.
 51. The process of claim 46 whereinthe liquid is contacted with the apparatus in a batch or continuousprocess.
 52. The process of claim 46 wherein the liquid is generallycontinuously or intermittently stirred.
 53. The process of claim 46wherein the liquid moves past the outer surface(s) in a generally plugflow.
 54. The process of claim 46 wherein the biofilm is maintained in astate of generally endogenous growth.
 55. The process of claim 54performed in a septic tank or shipboard system or to treat a wastewatertaken generally directly from one or more houses or businesses or partsof a ship.
 56. The process of claim 46 wherein the biofilm is maintainedat a thickness between 0.05 mm and 2 mm, more preferably between 0.1 mmand 1 mm.
 57. The process of claim 46 further comprising the steps ofmaintaining a least a portion of the biofilm so that its thicknessalternately increases and decreases, the biofilm increasing in thicknessover first periods of time and, between the first periods of time,reducing the thickness of the biofilm.
 58. The process of claim 57wherein the thickness of only a portion of the biofilm is reduced at atime.
 59. The process of claim 57 wherein the thickness of the biofilmis reduced by air scouring or agitating at least a portion of theliquid.
 60. The process of claim 57 wherein the thickness of the biofilmis reduced by contacting at least a portion of the biofilm with a secondliquid containing worms or other animals which digest the biofilm. 61.The process of claim 57 wherein the thickness of the biofilm is reducedby applying ozone to at least a portion of the biofilm from the lemenside of the fibers or from the outside of the biofilm to oxidize theportion of the biofilm and then maintaining the biofilm to digest theoxidized portion.
 62. The process of claim 61 wherein the thickness ofthe biofilm is reduced by introducing ozone gas into the port followedby supplying oxygen to the port.
 63. The process of claim 57 wherein thethickness of the biofilm is reduced by supplying air to the port whilethe liquid is removed from contact with the biofilm or provided at aloading of less than 0.1 kg CODs per kg MLSS per day to digest thebiofilm aerobically.
 64. The process of claim 57 wherein the thicknessof the biofilm is reduced by applying a control agent to at least aportion of the outer surface of the biofilm.
 65. The process of claim 64wherein the control agent is clean water.
 66. The process of claim 64wherein the control agent is heated clean water, preferably heated tobetween 40 and 60 C.
 67. The process of claim 64 wherein the controlagent is ozone gas.
 68. The process of claim 64 wherein the controlagent is an alkali solution with a pH between 8 and 13, more preferablybetween 9 and
 11. 69. The process of claim 64 wherein the control agentis an acid with a pH between 1 and 6, more preferably between 3 and 4.70. The process of claim 64 wherein the control agent is a second liquidand the second liquid is agitated or aerated while in contact with thebiofilm.
 71. The process of claim 64 wherein the biofilm is digestedaerobically after the control agent is applied.
 72. The process of claim57 wherein the thickness of the biofilm is reduced by draining theliquid away from contact with the biofilm.
 73. The process of claim 57wherein the thickness of the biofilm is reduced by stopping or reducingthe supply of oxygen to the port from time to time or periodically tocreate alternating aerobic and anoxic or anaerobic conditions in aportion of the biolfilm.
 74. The process of claim 57 wherein thethickness of the biofilm is reduced by physically removing a portion ofthe biofilm.
 75. The process of claim 74 wherein the biofilm isphysically removed by spraying it with a third liquid or scraping itwith a brush or scraper.
 76. The process of claim 57 wherein the liquidis removed from a portion of the biofilm while the thickness of thatportion of the biofilm is being reduced.
 77. The process of claim 46wherein the amount of oxygen supplied to the port is increased during aperiod of time when the CODs of the liquid is increased.
 78. The processof claim 46 wherein the liquid is periodically removed from the biofilmand replaced with a fresh batch of liquid and the supply of the gas iscontinued while the liquid is being removed, while the biofilm is not incontact with the liquid or while a fresh batch of liquid is beingreplaced in contact with the biofilm.
 79. The process of claim 46wherein the liquid, after being treated, has less than 10 mg/L ofsuspended solids and less than 50 mg/L of CODs.
 80. The process of claim46 operated in a two stage process wherein the first stage of theprocess reduces the CODs of the liquid to less than 300 mg/L, morepreferably to between 200 and 300 mg/L.
 81. The process of claim 46wherein the liquid, before treatment, has a CODs of 1000 mg/L or moreand the apparatus has a surface area for gas transfer to surface area ofattached biofilm of 1 or more, more preferably between 1 and
 10. 82. Theprocess of a claim 46 wherein the liquid, before treatment, has a CODsof 1000 mg/L or less and the apparatus has a surface area for gastransfer to surface area of attached biofilm of between 0.2 and 2.5. 83.The process of claim 46 wherein the liquid, before treatment, has a CODsof 300 mg/L or less and the apparatus has a surface area for gastransfer to surface area of attached biofilm of 1 or less, morepreferably between 0.1 and
 1. 84. A method for cutting the ends offibers in the apparatus of claim 18 comprising the steps of gluing apotting resin around the open or looped ends of a plurality of thefibers and then cutting through the resulting block of hardened resinand fibers.
 85. A method of producing an apparatus according to claim 18comprising the steps of and adhering spacers to the planar member(s)parallel to but displaced from the open ends of the hollow fibers, afirst edge of the spacers being nearer the ends of the hollow fibers anda second edge of the spacers being farther from the ends of the hollowfibers, inserting the planar member(s) into a header cavity, andapplying a potting resin over the second edge of the spacers extendingfrom the planar member(s) to walls of the header cavity.
 86. The processof claim 57 wherein the thickness of the biofilm is reduced at leastevery 10 days or after the biofilm has digested between 20 and 200 gramsof CODs per square metre of biofilm area since the last reduction. 87.The process of any of claims 46 operated as a batch process having stepsof draining the liquid from a tank containing the apparatus, thedraining step further comprising a step of draining a first part of theliquid containing settled solids to a first treatment system anddraining a second part of the liquid to a second treatment system.